Transformer bushing construction



July`2, 1963 A. J. SHABACK TRANSFORMER BUsHING CONSTRUCTION Filed April 28, 1959 INVENTOR, Anomaw J. SHHBACK Maj/ Arran/.fx

United States 3,096,392 TRANSFORMER RUSHING CNSTRUCTIN Andrew J. Shaback, Philadelphia, Pa., assigner, by mesne assignments, to H. K. Porter Company, Inc., Pittsburgh, Pa., a corporation of Delaware Filed Apr. 28, 1959, Ser. No. 809,494 12 Claims. (Cl. 174-31) This invention relates to transformer bushings, and more particularly relates to trans-former bushings of the high voltage type which connect the windings of a transformer housed within a tank or Casework to a terminal located exteriorly of the tank. Since in general the transformer Casework is maintained at .ground reference lpotential, the bushings which bring the high voltage potential of the cased transformer out through the casing wall to an exterior point must be capable of withstanding very high dielectric stresses without electrical breakdown and without giving rise to radio interference noise.

In the past, transformer bushings have been known which included essentially a hollow body of insulating material partially filled with an insulating oil and including a metallic tube extending centrally through the hollow portion of the insulator from top to bottom thereof, this metallic tube being connected at its lower end to the transformer high Avoltage Winding within the transformer housing, and being connected at its upper end to a terminal lying outside of they transformer housing. The hollow tube, of course, naturally passes through the transformer housing at some point intermediate its top and bottom, and it is in this region that the maximum dielectric stress is established. This known type of construction suffers from the serious disadvantage that under operating conditions heat generated by the transformer causes the metallic tube running through the bushing to tend to ex- -pand by lengthening. Since the tube is clamped at both its top and bottom ends, one of two highly unsatisfactory conditions frequently occur.

Firstly, the tendency toward expansion of the metallic tube places the insulator bushing in tension, and since most insulator bushings for such application are made of porcelain or other ceramic materials which are not particularly strong in tension the insulator bushing itself may crack and thereby cause breakdown of the insulating function performed by the bushing. Secondly, in the case where the insulator is able to withstand the tensile force, the metallic tube will tend to buckle, and the midpoint will ldeiiect outwardly resulting in a bowed shape of the metallic tube. The outward deflection of the tube in its central region moves the tube at this point closer to the edge of the hole in the transformer casing where the bushing passes therethrough and hence increases the dielectric stress in this region.

In many cases the increased dielectric stress due to the increased electrostatic field force which exists between the metallic tube within the bushing at high .potential and the transformer Casework at ground potential is suicient to cause electrical streamers in or on the insulating material therebetween resulting in corona discharge and giving rise to high frequency noise which radiates in all directions as though being broadcast by an antenna. This latter condition may, of course, be prevented by manufacturing the insulator bushing with a much greater diameter than would normally be needed to thereby keep the dielectric stress within tolerable limits for any condition which might arise. However, such a solution is no solution at all since the cost of such a lbushing will naturally increase with an increase in size, and moreover, in some applications an increased diameter insulating bushing may not .be permissible because of size considerations dictated by other factors.

My invention is directed toward eliminating the afore- 3,096,392 Patented July 2, 1963 'ice described high undesirable attributes of presently known high voltage bushings. Accordingly, it is a primary object of my invention to provide a novel insulating bushing of such construction as to prevent the insulator material Ifrom being placed in tension under operating conditions and hence preventing mechanicalf ailure of the insulator.

Another object of my invention is to provide a novel insulator bushing constructed 4in such fashion that the dielectric stress required to be withstood by the insulation of the bushing is maintained substantially constant and has no tendency to markedly increase during transformer operation.

A furtherv object of my invention is to provide a novel insulator bushing having the foregoing desirable characteristics which may be of relatively small diametral dimension in the region of highest dielectric stress as compared with insulator bushings capable of withstanding the same dielectric stress and made according to known practices.

Still another object of my invention is to provide a novel insulator bushing which under .conditions of transformer operation prevents the generation of radio frequency interference by preventing electrostatic breakdown of the insulator in the region of maximum dielectric stress.

The foregoing and other objects of my invention will become apparent from a careful reading of the yfollowing specification when taken in conjunction with the appended drawings, wherein:

FIGURE l is a side view of the novel transformer bushing according to my invention, some parts being shown in section and other parts in elevation;

FIGURE 2 is a cross-sectional view of the transformer bushing as Viewed along the lines 2 2 of FIGURE l;

FIGURE 3 is a cross-sectional view of the transformer bushing as viewed when taken alon-g the lines 3-3 of FIGURE 1;

FIGURE 4 is a fragmentary side view, partially in section, illustrating certain details of the internal construction of the transformer bushing with regard to the conductor system for leading the high voltage from the transformer winding outward to the external terminal at the top of the transformer bushing.

In these several `figures like elements are denoted by like reference characters.

I Turning now to an examination of the figures and primarily `and principally t-o FIGURE l, there will be seen an insulator bushing having a main body 10 form-ed from a suitable substance such as porcelain or other. ceramic material. Running centrally through the bushing from top to bottom is a central aperture 11 :and within which is disposed a hollow metal tube 17 through which extends a metallic ycable 24. The hollow metal tube 17 is threaded at its bottom end, Ias at 18, and is fixed in place within the bushing by 4the washers 19 and 7.2 which are disposed on either side of .a compressible rubber gasket or sealing ring 21. The washer 19 has extending inwardly from its top surface a plurality of ears 2t? which bear against the metal tube 17 and thereby prevent the washer 19 from being ldisplaced upwardly along the tube. The metal tube 17 is secured in place within the bushing by means of the nut 23 which is threaded on -to the threaded portion 18 of the tube 17. When the nut 23 is rotated so that it advances upwardly along the threaded portion 18 of the tube 17, the washer 22 is forced upwardly toward the washer 19 and hence compresses the gasketing material 21 so that the latter expands radially into sealing engagement with the inside wall of the aperture 11 extending through the insulator 16. Simultaneously the gasketing material 21 is expanded radially inwardly into sealing engagement with the surface of the tube 17. When now the central aperture 11 of the bushing 1i) is iilled with insulating oil,

this oil will be prevented from leaking out of the bottom of the bushing by virtue of the seal provided by the gasket 21.

The metallic cable 24 which extends through the hollow metal tube 17 is seen to be secured at its top `end to a metallic stud 26 :and this stud 26 is seen to pass through an insert 28 which is recessed into the top of the main body of the bushing. The stud 26 is preferably keyed into the insert 28, and the insert 28 is also preferably keyed into the main body of the insulator so that the stud 26 and insert 28 are held in non-rotatable fashion relative to each other and to the main body of the bushing. The stud 26 is threaded along its length and threadedly engages a metallic terminal casting 29 which may be rotated onto the threaded portion of the stud 26 and 'hence advance downward into seating engagement on the top of the insulator bushing. A gasketing washer 30 is placed between the undersurfaee of the terminal casting 29 and the top of the insulator bushing in Order to seal the bushing at the top and also act las a somewhat resilient cushion between the terminal casting and the bushing proper. As the terminal casting 29 is rotated, the stud 26 i-s pulled upward and ,tends to carry the cable 24 with it. It would, therefore, appear that the terminal 29 would not `become secured to the bushing but would merely continue to pull the cable 24 upward until the stud could no longer advance. However, this is not the case because the cable 24 is restrainingly engaged with the tube 17 in the region through which the sectional view o-f FIGURE 2 has been taken, and this inter-engagement is best seen in the showing of FIGURE 4 to which attention should now be directed.

FIGURE 4 shows `a fragmentary View of a portion of the tube 17 and the cable 24 wherein it will be observed that the cable 24 includes a short expanded diameter section 27 of suiicient thickness to firmly, frictionally lengage the inside ysurface of the hollow tube 17. This frictional engagement may also be clearly seen in the cross-sectional view of FIGURE 2 where it will be observed that no clearance exists between :the expanded portion 27 of the cable 24 and the inside of the tube 17. Although so illustrated, it will still be appreciated that some clearance does still exist between the expanded portion 27 of the cable 24 and the inside surface of the tube 17 in the regions between the strands forming the outside surface of the cable. The outside lcable strands themselves are, however, in frictional engagement with the tube 17 as previously mentioned. Since the cable 24 and the hollow tu-be 117 are effectively locked together by the expanded cable portion 2.7, as the terminal 29' is rotated to draw the stud 26 upwardly thereinto, the cable 24 will be drawn taut above the lregion where it is locked to the hollow tube. Thereafter, continued turning of the terminal 29 will cause the cable 24 and tube 17 to tend to move upwardly together. This tendency toward upward movement of the cable and the tube as a unit is opposed by the action of the expanded gasket 21 at the bottom of the bushing, so that in effect the terminal 29 will be drawn downwardly into the -desired seating engagement with the top of the bushing.

From the foregoing, it will be appreciated that the hollow tube 17 and cable 24 are securely centrally fixed within the hollow portion 11 of the bushing. It will be further observed that the top of the hollow tube 17 stops short of the top of the 4bushing and hence `during operation of the transformer when the tube 17 and Icable 24 become heated due yto the transformer temperature rise, and hence tend to expand, the cable 24 being flexible will absorb the dimensional change without thereby placing the body of the insulator in tension and hence will eliminate lany possibility of insulator breakage.

In use, the hollow central region 11 extending through the insulator bushing is filled with insulating oil to a level somewhat below the open top of the hollow tube 17, so that when a temperature rise takes place and the oil in the insulator bushing expands, the insulating oil may move upward to the top of the hollow tube 17 `and may then begin to move downward thereinto. As before mentioned, even though the expanded portion 27 of the cable '24 is frictionally engaged with the inside surfaces of the hollow tube 17, there remains sufficient room between the cable strands to allow any oil flowing downward through the hollow tube 17 to pass the expanded portion of the cable and continue downward int-o the tube. The oil so owing downward into the hollow tube 17 will, of course, no-t continue to flow down the tube into the transformer tank itself since in practice a goodly portion of the bottom end of the bushing` lies below the level of the insulating oil within the transformer casing and surrounding the transformer windings.

When the oil temperature is decreased, perhaps due to shutting down of the transformer or running the transformer under lighter load, :the oil in the hollow tube 17 will risc therein and will flow backwards into the hollow central portion of the bushing as the oil level therein drops. Thus, the hollow tube 17 in addition to maintaining the cable 24 centered within the bushing, also provides a breathing function for the insulating oil contained within the insulator bushing central aperture. Finally, the physical engagement between the cable 24 land the hollow tube 17 eifeet-ed by the expanded Icable portion 27 electrically connects the hollow tube 17 t-o the cable 24 and hence the tube 17 assumes [the high voltage potential of the cable, such high voltage being applied to the cable 24 from the transformer winding via the terminal 25 connected to the cable lower extremity.

The high voltage bushing as just described may be securely mounted to the transformer casing in the manner illustrated in FIGURE l. In FIGURE l, it will be seen that Ithe lower non-corrugated portion 05E the bushing extends downwardly through an aperture in the transformer casing 112 with a gasket 13 intervening the outer sunface of the transformer casing and the undersurface of the lowest insulator bushing corrugation. Surrounding the insulator bushing shank portion and lying partly within a circumferentially extending recess thereinis a ring 15 which supports the collar 14. The collar 14 is provided with threaded holes spaced at suitable intervals about its Harige-like periphery and threaded through these holes are Ea plurality of set screws or bolts 16. When the set screws 15 are rotated clockwise as viewed from below they will be advanced upwardly into bearing engagement with the undersurface of the transformer casing 12 and will pull the bushing downward so that the gasket ring 13 is clamped securely between the bushing and the transformer casing and therefore seal the transformer casing.

Understanding now, as previously described that tlie cable 24 is `free to llex slightly and thus strain relieve the insulator bushing itself, it will now also be appreciated that the hollow tube 17 will be maintained substantially in its central position so that the spacing between the outer surface of the hollow tube 17 and the transformer casing 12 will remain substantially constant regardless of temperature elevations due to heating of the transformer. Since this spacing does not decrease, due to the absence of bending orf the tube 17, the electrostatic field remains substantially constant and there is no increase in ydielectric stress thnough the body of the insulator bushing.

Although my invention has been described in connection with a particularly illustrated embodiment thereof, it will be appreciated that various modifications will occur from time to time to those persons normally skilled in the art without departing from the essential spirit or scope of my invention, and it is therefore intended to claim the ysame broadly as well as specifically as indicated by the appended claims.

What is claimed as new and useful is:

1. An insulator bushing comprising a body of insulator material having a longitudinally extending aperture therein, a metallic cable extending centrally through said aperannessa ture from one end to the other, a metallic tube Isurrounding said cable and extending centrally through said aperture irom a point adjacent said one end to a point there- Within short of said other end, terminal means connected to said cable at said other end, means for securing said cable to said tube, and means for securing said tube ccntrally within Isaid longitudinally extending aperture.

2. The insulator bushing according to claim 1 wherein the aperture extending longitudinally through the body of insullator material is centrally located therewithin, extends from one end to the other, and is of larger cross-sectional area than the cross-sectional area of said tube.

3. The insulator bushing according to claim 2 wherein said cable is generally of smaller diameter than the hole extending through the metallic tube, and the means for securing said cable to said tube comprise a portion of said cable of expanded diameter, said expanded diameter cable portion throughout its length engaging the inside wal-l of said tube.

4. The insulator bushing according to claim 3 wherein said means for securing said tube centrally within said longitudinally extending aperture also comprise means Vfor sealing said aperture against the iiow of fluid contained therewithin out of said one end.

5. The insulator bushing according to lclaim 2 wherein said means for securing said tube centrally within said longitudinally extending aperture also comprise means for sealing said aperture against the flow ot fluid contained therewithin out of said one end.

6. The insulator bushing according to claim l wherein said cable is generally of smaller diameter than the hole extending through the metallic tube, and the means for securing said cable to said tube comprise a portion of said cable of expanded diameter, said expanded diameter cable portion throughout its length engaging the inside wall of said tube.

7. The insulator bushing according to claim 6 wherein said means for securing said tube centr-ally within said longitudinally extending aperture also comprise means for sealing said aperture against the flow of fluid contained therewithin out of said one end.

8. The insulator bushing according to claim 1 wherein said means for securing sai-d tube centrally within said longitudinally extending aperture also comprise means -or sealing said aperture against the ilow lot iluid contained therewithin out of said one end.

9. An insulator bushing comprising an elongated body of insulating material having a top end and a bottom end, an open-ended cylindrical passage extending centrally 1ongitudinally through said elongated body of insulating material from said top end to said bottom end, a recess in said top end coaxial with said passage and of greater cross-sectional area than said passage, an insulator insert non-rotatably seated in said recess and having a top sur face flush with said top end of said elongated body of insulating material, a stud having a head non-rotatably seated in said insert and having a threaded end extending outward therethrough in coaxial alignment with said passage, an apertured gasket seated on top of said insert with said stud threaded end projecting through said gasket aperture, said gasket being of greater cross-sectional area than said recess, a terminal Icasting containing an aperture threaded complementally to said stud threaded end and having a ilange portion adapted to seat lirnily down upon said gasket and when so seated causing said gasket to seal the top end of said elongated body, a metallic cable connected to said stud head and extending centrally downward through said cylindrical passage and out through the bottom end, an open-ended metallic tube surrounding said cable and of .generally greater inside diameter than the diameter of said cable but oi lesser outside diameter than the diameter of said cylindrical passage, said tube extending from the bottom end of said passage upward therethrough and terminating below smd stud head in underlying spaced relation thereto, said cable including an expanded diameter portion which firmly engages the inside wall of said tube proximate said tubes upper terminus, said tube proximate its lower terminus being centrally secured within said `cylindrical passage by a longitudinally-compressed radially-expanded annular sealing ring, the outer periphery of said ring being pressed firmly circumferentially against the wall 'of said cylindrical passage and the inner periphery of said ring being pressed firmly circumierentially against the outer wall of said tube, whereby, when said terminal casting is rotatably threaded onto said stud said casting is pulled downward and compresses said gasket to seal the top end of said bushing.

l0. The insulator bushing according to claim 9 further including a quantity oi insulating oil partially filling said cylindrical passage external to said tube from the bottom of said passage above said sealing ring to a point somewhat below the open top of said tube.

1l. The insulator bushing according to claim l0 further including means associated with the exterior of said elongated body of insulating material for securing said body to a transformer case with a portion of the bushing :extending downward inside the transformer case and a portion ot the bushing extending upward outside of the transformer case, the insulating oil partially filling said cylindrical passage lying both in the bushing portions extending inside and outside of the transiormer case.

12. The insulator bushing according to claim 9 further including means associated with the exterior of said elongated body of insulating material for securing said body to a transformer case with a portion of the bushing extending downward inside the transformer case and a portion of the bushing extending upward outside of the transformer case.

References Cited in the file of this patent UNITED STATES PATENTS 1,887,010 Cage Nov. 8, 1932 1,983,335 Austin Dec. 4, 1934 2,082,046 Brandt June 1, 1937 2,249,862 Skvortzo July 22, 1941 2,658,934 Field Nov. 10, 1953 2,859,271 Johnston Nov. 4, 1958 FOREIGN PATENTS 512,217 Canada Apr. 26, 1955 512,252 France Oct. 8, 1920 792,460 Great Britain Mar. 26, 1958 

1. AN INSULATOR BUSHING COMPRISING A BODY OF INSULATOR MATERIAL HAVING A LONGITUDINALLY EXTENDING APERTURE THEREIN, A METALLIC CABLE EXTENDING CENTRALLY THROUGH SAID APERTURE FROM ONE END TO THE OTHER, A METALLIC TUBE SURROUNDING SAID CABLE AND EXTENDING CENTRALLY THROUGH SAID APERTURE FROM A POINT ADJACENT SAID ONE END TO A POINT THEREWITHIN SHORT OF SAID OTHER END, TERMINAL MEANS CONNECTED TO SAID CABLE AT SAID OTHER END, MEANS FOR SECURING SAID CABLE TO SAID TUBE, AND MEANS FOR SECURING SAID TUBE CENTRALLY WITHIN SAID LONGITUDINALLY EXTENDING APERTURE. 